It is desirable for many applications to have a tunable integrated circuit capacitor. For example, communication devices have a need for voltage controlled oscillators (VCO). Micro-electro-mechanical systems (MEMS) provide a way to construct a variable capacitor within an integrated circuit. Conventionally, a parallel-plate micromachined variable capacitor is fabricated by forming a fixed capacitor electrode on a substrate, and forming a movable capacitor electrode that is held in place parallel to the fixed capacitor electrode by a system of springs. By applying a control voltage between the fixed and movable capacitor electrodes, the movable capacitor electrode is pulled towards the fixed capacitor electrode due to the electrostatic force between the two capacitor electrodes. Because the capacitance of a parallel-plate capacitor is inversely proportional to the distance between the electrodes, the capacitance is altered by changing the distance between the electrodes. In addition to the control voltage, a signal voltage is applied to the capacitor electrodes. The signal voltage is the voltage applied to the capacitor for the purposes of the circuit in which the tunable capacitor resides. Typically, the control voltage is a large DC signal, whereas the signal voltage is a small AC signal. However, the control voltage can be an AC signal with a frequency above the frequency at which the movable capacitor electrode/spring system has a significant mechanical response.
A problem with conventional parallel-plate micromachined variable capacitors is keeping the distance between the movable and fixed capacitor electrodes uniform across the opposed surfaces of the capacitor electrodes. The movable capacitor electrode tends to bend. For example, conventionally, the movable capacitor electrode is supported by springs that are anchored to the substrate. Temperature variations cause the movable capacitor electrode and the substrate material to expand by different amounts. This can cause the movable capacitor electrode to bend either convexly or concavely. Consequently, the distance between the two capacitor electrodes is non-uniform across the surface of the electrodes. Moreover, because the movable capacitor electrode can bend either convexly or concavely, the capacitance of the device either decreases or increases unpredictably. Further, bending can occur during fabrication of the capacitor, (due to variations in the internal stresses of the deposited materials,) resulting in a deformed movable capacitor electrode. A deformed capacitor electrode will not have the capacitance for which it was designed.
The amount by which the capacitance of conventional parallel-plate micromachined variable capacitors can be varied is limited. Moreover, such capacitors are susceptible to a snap-together effect in which the movable capacitor electrode snaps into contact with the fixed capacitor electrode if the two capacitor electrodes get too close together. That is, when controlling the movable capacitor electrode, the electrostatic force due to the control voltage works against the force from the system of springs connected to the movable capacitor electrode. When the capacitor electrodes get too close together, the electrostatic force overwhelms the force from the springs, and the capacitor electrodes snap together. This is because the electrostatic force increases proportional to 1/x2, whereas the spring force increases proportional to Δx, where “x” is the distance between the capacitor electrodes and Δx is the distance moved. Snap-together typically occurs when the movable capacitor electrode has moved about ⅓ of the initial gap, at which point the capacitance increase is about 50 percent. Thus, conventional parallel plate variable capacitors typically have a maximum tuning range of about 1.5:1 between the capacitance at snap-together and the initial minimum capacitance. However, because the movable electrode could be deformed and also has a tendency to bend, snap-together can occur even if the movable capacitor electrode has moved less that ⅓ of the initial gap. The snap-together effect prevents achieving a larger change in capacitance that would otherwise be achievable if the capacitor electrodes could be brought closer together. Moreover, if the capacitor electrodes do snap together, a high current can flow that could damage other components in the integrated circuit.
Thus, one problem with conventional parallel-plate variable capacitors is that the movable capacitor electrode is subject to bending, so that the capacitance changes unpredictably. Another problem with conventional parallel-plate variable capacitors is that the range of capacitance is too limited due to the snap-together effect. Still another problem with conventional parallel-plate variable capacitors is that components can be damaged if the capacitor electrodes short together.